The lining of blood vessels is capable of releasing a substance that causes relaxation of smooth muscle. The substance is called endothelium- derived relaxant factor. L-arginine, a normal constituent of the diet, is a precursor for synthesis of endothelium-derived relaxant factor. Because dysfunction of release of endothelium-derived relaxant factor can predispose to clot formation (thrombosis), we investigated the effect of administration of L-arginine on platelet function. The aggregation of platelets, a measure of the ease with which blood can clot, was suppressed by L-arginine administration. This suggests that L-arginine supplementation may be beneficial in patients with coronary artery disease who have predisposition to clot formation. This will be investigated in future studies.